The use of optical measuring devices, for example, in real-time detection, diagnosis and imaging of diseases can require a regular calibration and characterization of such optical measuring devices. The calibration of optical measuring systems can be effected with standards, such as, for example, certified radiation-intensity-per-unit-area standards or a spectrally corrected standards where an instrument-independent emission and/or scatter spectra can be used. The availability of such standards is very important for the long-term instrument stability and reliability.
Generally speaking, the measurement of reflectance or fluorescence or Raman signals plays a great role in biomedical applications, for example for the early diagnosis of disease-specific changes on the molecular level. American Society for Testing and Materials (ASTM) has established a series of Raman shift frequency standards (ASTM E 1840) that can be used in calibration of Raman spectrometers. Typically, Raman shifts frequencies of the following compounds are included in the standard: Naphthalene; benzene; sulfur; toluene; acetaminophen; benzonitrile; cyclohexane; and polystyrene.
Spectroscopy systems such as a Raman spectroscopy system or fluorescence spectrometer can be limited by a poor signal to noise ratio which may impede the spectral measurements and reliability of such measurements especially for in vivo real time medical procedures. The low signal to noise ratio is a consequence of the low strength or absence of an optical signal coming from the target tissue, a high level of background noise and poor sensitivity and specificity of the detected optical signal.
Therefore, there is a need for reliable, accurate and simple method and apparatus for calibration, characterization and reliability check of the spectral measuring systems such as Raman, fluorescence and reflectance spectroscopy systems for a long-term system stability and reliability.